The present invention relates to volume control of audio transducers in general, and more particularly, to a volume control circuit including a driver circuit for exciting an audio transducer with a pulse rate signal to render an audio signal at a desired volume and frequency corresponding to the pulse rate signal, and a control circuit selectively activated to pulse modulate the excitation of the audio transducer, thereby altering the volume of the audio signal rendered thereby.
The techniques of volume control are many and generally find application in general purpose radio receivers for altering the volume of the audio signal over the tunable frequency range thereof. General purpose radio receivers usually include sophisticated audio amplifier circuits for implementing two or four quadrant amplification. Typically, volume control is implemented by varying the gain of one or more of the amplifier stages. This variable gain technique generally requires complicated hardware, and because the audio amplifiers are operating in the linear region, the technique requires a great deal of operating power.
For more special purpose radio receivers, like paging receivers, for example, the audio transducer is driven at one or more fixed frequencies, generally on the order of 2 Khz or so, to provide an audio alert to the user of an incoming page signal. The paging variety of receivers are, for the most part, portable and battery powered. Thus, the criteria for implementing a volume control in a paging receiver is quite different from that of a general purpose tunable audio band radio receiver. This criteria includes the ability to adjust the volume of the audio signal (1) to be heard by the user above the sounds of the surrounding environment, (2) to be aesthetically pleasing (e.g. not too loud and free of whistles and/or buzzes) to the user and others in the close proximity, and (3) to reduce the drain on the battery source when it approaches the end of its useful battery life.
Some present volume control techniques include varying the frequency of the audio drive signal, but this causes an undesirable change in audio frequency as well. Another technique involves a manually controlled, mechanically switched resistor in series with the audio transducer to switch in and out a current limiting load resistance, but since the mechanical switch is manually operated and must carry the drive current of the audio transducer, it must be of a high quality and thus, expensive and cannot be electronically operated. Yet another technique of volume control involves the pulse width modulation of the audio drive signal, but this technique introduces harmonics in the audio range which affects the total quality of the audio signal with buzzing and whistling, for example. The pulse width modulation technique also requires sophisticated circuitry which increases part count, battery drain and cost. Still another technique, described in the U.S. Pat. No. 4,382,251 issued to Fujisawa on May 3, 1983 and assigned to Casio Computer Co., involves AM variation of the tone frequency of the audio drive signal, which technique has been found suitable for the piezo-electric buzzer type audio transducers.
The present invention proposes to avoid the drawbacks of the above mentioned volume control techniques and provide an inexpensive, low current drain circuit for controlling volume over a multiple of volume levels without harmonic generation in the audio range.